The present invention generally relates to a frequency converter, and more particularly to a frequency converter which outputs a clock signal having a stable and accurate frequency.
Such a frequency converter is used for generating a reference clock signal of a phase-locked loop (PLL) circuit widely used for controlling a rotational speed of a stepping motor, generating a synchronous signal for a communication system and generating a synchronous signal for a digital signal processing system.
Conventionally, a quartz-crystal resonator is used for constructing a system requiring a highly accurate and stable frequency, which quartz-crystal resonator oscillates at a specific frequency required by the system. For example, in a personal computer, a quartz-crystal resonator having a specific frequency such as 16 MHz, 20 MHz or 33 MHz is used according to a performance of a microprocessor used in the personal computer. A quartz-crystal resonator having a specific frequency is also used for a microcontroller widely used in an industrial field.
In an application which does not require a highly accurate and stable frequency, a ceramic resonator is used instead of the quartz-crystal resonator. The quartz-crystal resonator is also used for communication apparatus.
Generally, in the above-mentioned applications, particular frequencies are mostly used, and thus quartz-crystal resonators having specific frequencies are commercially available.
In a case where one quartz-crystal resonator is used for a clock signal having a specific frequency and when a different clock signal other than the specific frequency is needed, the different clock signal can be obtained by means of frequency conversion performed on the clock signal having the specific frequency. The frequency conversion is generally performed by a counter. For example, a programmable counter No. 8253 is widely used for generating a clock signal having a different frequency. Data for setting an operational mode and data for a frequency-dividing ratio are input to the programmable counter, and a clock signal used in a CPU is also input to the programmable counter so that a clock signal having a different frequency but the same accuracy and stability as the clock signal supplied by the CPU is obtained. Of course, a different clock signal for a processing unity which is not associated with the CPU or for a local communication may be generated by using another oscillator exclusively used for such applications.
When an accurate rotation of a motor is required, a PPL control system is widely used. In the PPL control system, the rotation of a motor is controlled so that an output of an encoder connected to a shaft of the motor synchronizes with an output of an oscillator using a quartz-crystal resonator. The motor controlled by the PPL control system is used as a drive motor in various apparatuses which require an accurate rotational speed. Those apparatuses include a hard disk drive, a floppy disk drive, a driving unit of a photosensitive drum of a printer or a copy machine, a driving unit of a polygon mirror of a laser scanner and a card feeder of a magnetic card reader. These apparatuses require a specific rotational speed of a motor, and thus various quartz-crystal resonators each having different frequencies are used.
As mentioned above, various quartz-crystal resonators other than that having standard frequencies are needed for various applications. Accordingly, it may take a long time to obtain a quartz-crystal resonator having a special frequency. Additionally, when a number of the resonators to be manufactured in a lot is small, a manufacturing cost per one resonator is increased.